There are known vehicle lamps in which semiconductor light-emitting elements such as LEDs (Light Emitting Diodes) are used as semiconductor light sources. A lighting controller for controlling the lighting of the LEDs is mounted on vehicle lamps of this kind.
The foregoing lighting controller includes a single switching regulator, series regulators corresponding to a multiple LEDs, and protection control circuits corresponding to the individual series regulators. (See, e.g., Japanese Published Unexamined Patent Application No. 2006-103477.)
The single switching regulator has a capacitor, a transformer, a diode, a first NMOS (Negative Channel Metal Oxide Semiconductor) transistor and a control circuit, acting as current supply means for supplying a driving current to LEDs.
Each of the series regulators includes a comparison amplifier, a second NMOS transistor, a shunt resistor and a reference power source for generating a reference voltage. Each of the second NMOS transistors is connected in series to an LED together with the shunt resistor acting as a switching element. The comparison amplifier compares a reference voltage entered into a non-inverting input terminal (positive input terminal) with a voltage drop (decreased voltage of shunt resistor) input into an inverting input terminal (negative input terminal), generating a gate voltage (control signal) depending on the comparison result, applying the gate voltage to a gate of the second NMOS transistor to control the second NMOS transistor so as to be subjected to ON/OFF operation, thereby gaining control so that a specified current will flow through each of the LEDs.
If a current flowing through any one of the LEDs is lower than the specified current, the gate voltage of the second NMOS transistor increases. If the gate voltage of any one of the second NMOS transistors increases, the control circuit controls the ON/OFF operation to the first NMOS transistor so as to increase the output voltage of the single switching regulator. Further, when the gate voltage of all the second NMOS transistors decrease to a level of threshold voltage, the control circuit controls the switching operation of the first NMOS transistor so as to decrease the output of the single switching regulator.
Each of the protection control circuits controls the operation of a respective one of each second NMOS transistor so as to be safe in response to an abnormal gate voltage due to the applied voltage of each LED or the output of each comparison amplifier. Each of the protection control circuits includes a first Zener diode, a second Zener diode, a diode, a CR circuit, a PNP transistor and an NPN transistor. The cathode side of the first Zener diode is connected to the output side of the comparison amplifier, the cathode side of the second Zener diode is connected to a drain of the second NMOS transistor, and the anode side of the diode is connected to a gate of each of the second NMOS transistors.
The first Zener diode detects the presence or absence of an abnormal gate voltage due to the output of the comparison amplifier. Upon detection of the abnormal gate voltage by the first Zener diode, the operation of each of the second NMOS transistors is controlled.
For example, when an LED is broken as a result of wiring to cause an abnormal opening of the LED, no current will flow through the second NMOS transistor. However, the comparison amplifier provides control so that a specified current will flow through the second NMOS transistor, by which gate voltage of the second NMOS transistor gradually increases due to the output of the comparison amplifier and the second NMOS transistor is saturated in the ON state of operation. Further, when, due to the output of the comparison amplifier, the gate voltage is higher than a Zener voltage of the first Zener diode, a Zener current will flow through the first Zener diode, and an electric charge is accumulated in the capacitor until the elapse of time determined by a time constant of the CR circuit.
When the voltage of both ends of the capacitor is higher than the threshold voltage of the NPN transistor after the elapse of time determined by the time constant, the NPN transistor is in an ON state. Then, with a decrease in potential of a collector of the NPN transistor, the PNP transistor is in the ON state, and current will also flow through the diode. The gate voltage of the second NMOS transistor decreases, and the second NMOS transistor connected to an LED at which an abnormality takes place is in the OFF state.
On the other hand, the second Zener diode monitors the presence or absence of a voltage applied to the second NMOS transistor, that is, an abnormal drain voltage, detecting the abnormality with an increase in drain voltage. For example, when a short circuit occurs between the anode and cathode of an LED, the voltage on both ends of the LED is 0V. Therefore, a drain-source voltage of the second NMOS transistor connected to the LED at which an abnormality take place rises to a greater extent than normal, and the drain-source voltage is higher than a Zener voltage of the second Zener diode. In this instance, Zener current will flow through the second Zener diode, and an electric charge is accumulated in the capacitor until the elapse of time determined by the time constant in the CR circuit.
When the voltage of both ends of the capacitor is higher than the threshold voltage of the NPN transistor after the elapse of time determined by the time constant, the NPN transistor is in the ON state, and the PNP transistor is in the ON state. At the same time, current flows through the diode, the gate voltage of the second NMOS transistor decreases, and the second NMOS transistor is in the OFF state.
Specifically, when an LED is broken as a result of wiring, for the protection of LEDs and circuit elements of series regulators, each of the second NMOS transistors is controlled by each of the protection control circuits so as to be turned OFF. Further, when a short circuit takes place between the anode and cathode of an LED, for the protection of LEDs and circuit elements of series regulators, each of the second NMOS transistors is controlled by a respective one of the protection control circuits so as to be turned OFF.
Where an abnormality takes place as the result of a short circuit between the anode and cathode of an LED, while the abnormality is generated, power is consumed uselessly because the second NMOS transistor continues to consume power. Therefore, in order to sustain the useless consumption of power, it is preferable to halt operation of the series regulator instantly.
On the other hand, where an abnormality takes place by the opening of an LED, the abnormality is to the result of a poor contact of an output wiring or the like. Since an abnormality of this type does not damage a circuit element or lead to smoking and the like, it is preferable to provide such control that time until determination of the abnormality (time determined by the time constant of the CR circuit) is lengthened to make the abnormal detection less sensitive.
However, in the foregoing known technology, where an abnormality is detected as the result of the opening of an LED, after the elapse of time determined by the time constant of the CR circuit, the second NMOS transistor is turned OFF to halt operation of the series regulator. Where an abnormality is detected as the result of a short circuit between the anode and cathode of an LED, after the elapse of time determined by the time constant of the CR circuit, the second NMOS transistor is turned OFF to halt operation of the series regulator. Therefore, the duration from judgment of the abnormality to the halted operation of the series regulator is determined by the time constant of the CR circuit regardless of the type of abnormality.
As a result, in the foregoing known technology, it is difficult to determine an abnormality in an appropriate time depending on the type of abnormality of the LED. Operation of series regulators cannot be halted immediately after the elapse of an appropriate time depending on the type of abnormality, when the abnormality is generated. Thus, there is posed a problem that a circuit element may fail depending on the time until the halted operation of a series regulator.